This invention relates to circuit provisions for damping resonant ringing in an electrical circuit, and more particularly to a switched RC circuit connected with a resonant circuit to damp ringing.
In power devices such as DC-DC converters, abrupt voltage changes can result in transient ringing in the primary and secondary circuits of a transformer. For example, for a regular DC-DC converter, the rectification in a power transformer secondary uses one or more ultra fast or Schottky rectifiers. The leakage inductance of the transformer may interact resonantly with the reverse recovery current and/or with the junction capacitance of the output rectifier diode(s). The leakage inductance and the junction capacitance together with all the parasitic inductances and capacitances of the layout determine the resonant frequency. This resonant circuit has low losses. As a result, many cycles of ringing will occur after the first spike. Also, the ringing generates an overvoltage that constitutes the maximum reverse voltage across the rectifier. This can exceed the diode maximum voltage rating, or it may require larger, more expensive components than would be needed in the absence of such overvoltage. The ringing is present in the current flowing in the secondary side as well. The transformer then reflects the ringing to the primary side of the circuit. This can affect a current sense signal used by the controller when it is located in primary side and the converter works in current mode control. To avoid these problems, the ringing must be damped. However, the damping should be optimized since an excessive damping will increase the switching time and switching losses will become more significant.
FIG. 1 illustrates, for purposes of explanation, what probably is the most common circuit 25 used to suppress the voltage transient spikes. This is a typical RC snubber. It is a serial RC circuit 26 connected in parallel with a switching element shown for purposes of explanation in FIG. 1 as a simple switch SW. It can, however, be a discrete or integrated semiconductor switching device. In the absence of the snubber, when the switch abruptly opens the ringing occurs in the resonant circuit 28.
The value of the snubber capacitor will help define the resonant frequency of the circuit. This is another advantage because this frequency will be less dependent on the parasitic capacitances of the switching element or circuit layout and will be dictated mainly by the value of the snubber capacitor. This helps with EMI filtering by limiting the high frequency harmonics.
The snubber 25 is dissipative. Transferring the energy stored in the leakage and parasitic inductances to the snubber capacitor, a fraction of it is dissipated across the snubber resistor. Thus the snubber absorbs some energy at every voltage transient across the switch SW. The effect is the damping of the parasitic ringing at the moment when the switch opens. To properly damp the ringing, the value of the resistor is important. It should be close to the characteristic impedance of the parasitic resonant circuit.
The main disadvantage of this usual snubber 25 is the power lost cycle by cycle to charge the capacitor of the snubber from zero to the maximum value of the overvoltage across the switch. This energy is proportional to the value of the capacitor, the switching frequency and the square value of the voltage swing on the capacitor. Depending on the value of the voltage swing, these losses may become significant for power devices such as high efficiency DC-DC converters.
FIG. 2 schematically illustrates the standard RC snubber in the context of the equivalent circuit of a DC-DC converter shown as Li is the leakage inductance of a power transformer primary binding. An ultrafast or Shottky diode D2 serves as the output rectifier. The snubber circuit consists of a series-connected capacitor 30 and resistor 32 connected in parallel with the diode D2. An input approximating a square wave is applied at Vi. Power transformer inductance on the output side is represented at Lo and an output voltage Vo appears across the output capacitor Co.
In the single ended forward topology of FIG. 2, it is the free wheeling diode D2 that is the switching element that causes ringing requiring a snubber. When the diode D2 opens, ringing occurs as described above. FIGS. 3 and 4 show the voltage wave form across the diode D2 before and after application of the snubber. Vc the voltage across the capacitor 30 follows VD2. Each time VD2 ramps up, capacitor C charges. Each time VD2 ramps down, capacitor C discharges completely.
For efficient power device operation, it would be beneficial, where a snubber is used to damp ringing, to reduce the losses that occur as a result of current flow in the RC snubber circuit during the charging and discharging of the capacitor.